It would appear that, in the future, new battery systems will be used both in stationary applications, such as wind turbines, in motor vehicles in the form of hybrid or electric motor vehicles and in electronic appliances, such as laptops or mobile telephones, with very stringent requirements being placed on said battery systems in respect of reliability, safety, performance and life.
In vehicles with an at least partially electric drive, electrical energy stores are used in order to store the electrical energy for the electric motor which assists the drive or acts as drive. In vehicles of the most recent generation, in this case so-called lithium-ion batteries are used. These batteries are distinguished, inter alia, by high energy densities and an extremely low level of self-discharge. Lithium-ion cells have at least one positive and one negative electrode (cathode and anode, respectively), and the lithium ions (Li+) can be reversibly intercalated or deintercalated again.
FIG. 1 shows how individual battery cells 10 can be combined to form battery modules 12 and then batteries 14. This is performed by the poles of the battery cells 10 being connected in parallel or series (not illustrated). In this case, by definition, a battery module 12 or a battery 14 comprises at least two battery cells 10, wherein the terms battery 14 and battery module 12 are often used synonymously. The electrical voltage of a battery 14 is, for example, between 12 and 750 volts DC.
For authorization for transport and for use in motor vehicles, various tests are implemented on battery cells, for example lithium-ion battery cells. Inter alia, so-called abuse tests are also implemented in order to be able to assess the response of the battery cells under extreme situations, such as a traffic accident, for example.
In order to diminish the consequences of some abuse tests, mechanisms are installed in the battery cells which interrupt a current flow into the battery cell when the cell-internal pressure increases owing to abuse of the battery cell. For example, current interruptive devices (CID) or overcharge safety devices (OSD) are known.
U.S. Pat. No. 6,497,978 B1 discloses a possible embodiment of a mechanical current interruption. A safety mechanism is installed between the positive pole of a cylindrical battery cell and the positive electrode, which is electrically conductively connected to the positive pole. If the pressure within the battery cell increases, first a cover within the battery cell is deformed, as a result of which the electrically conductive connection between the positive pole and the positive electrode is interrupted. When the pressure further increases, battery gases can escape into the open air via a burst film.
JP 5062664 A discloses another variant of a mechanically activated safety device. The likewise cylindrical battery cell has an electrically conductive membrane at the cell cover of its positive pole. This membrane can expand in the event of a pressure increase within the battery cell and thereby makes contact with an overhang fastened on the battery cell housing, which overhang, as is the battery cell housing, is at the potential of the negative pole. By virtue of the contact of the membrane with the overhang, the battery cell is short-circuited as a result of which further overcharging is stopped, for example.
FIG. 2 shows a similar mechanism of an overcharge safety device using the example of a prismatic battery cell 10. Said battery cell comprises a likewise electrically conductive membrane 22, which is integrated in the electrically conductive battery cell housing 16 and is curved inwards in the fault-free state. Furthermore, the battery cell 10 comprises a pole 24 (in this case the negative pole) which is electrically insulated from the battery cell housing 16 and a pole 25 (in this case the positive pole) which is electrically conductively connected to the battery cell housing 16. The electrical insulation between the insulated pole 24 and the battery cell housing 16 is ensured via an insulator 28, whereas the connected pole can be, for example, part of the battery cell housing 16 or inserted therein. If the battery cell 10 is being charged, a charge current IC flows via the positive pole into the chemically active part 18 of the battery cell 10.
If the pressure within the battery cell 10 now increases, for example as a result of overcharging of the battery cell 10, the membrane 22 curves outwards, as illustrated in FIG. 3, and comes into electrically conductive contact with an overhang 26 on the insulated pole 24 of the battery cell 10. As a result, the two poles 24, 25 are electrically conductively connected to one another. The resistance of this electrically conductive connection is sufficiently low for an overcharge current IDC to no longer flow through the chemically active part 18, but through the battery cell housing 16, the membrane 22 and the overhang 26.
At the same time, however, a short circuit of the battery cell 10 via the two poles 24, 25 results. A short-circuit current ISC flows via the membrane 22 and the cell housing 16 and could in the process destruct the membrane 22. In order to prevent this, a fuse 20 is installed between the chemically active part 18 and one of the poles 24, 25, in this case the positive pole, which fuse interrupts the short-circuit current ISC before it can destroy the membrane 22.